Russia's armed forces are gearing up for Vostok-18, or East-18, a massive military exercise in the country's far east from Sept. 11-15, 2018.

Defense Minister Sergei Shoigu said in August 2018 that about 300,000 troops and 1,000 aircraft would participate, using all of the training ranges in the country's central and eastern military districts. Russia's Pacific and Northern fleets and its airborne forces are also expected to join.

Shoigu said 2018's iteration of the Vostok exercise would be "unprecedented in scale, both in terms of area of operations and numbers of military command structure, troops, and forces involved."


But the size of the forces involved is not the only feature that has turned heads.

Forces from China and Mongolia also plan to take part. Beijing has said it will send about 3,200 troops, 30 helicopters, and more than 900 other pieces of military hardware.

China's Defense Ministry said the drills were meant to strengthen the two countries' strategic military partnership and increase their ability to respond to threats and ensure stability in the region.

The Kremlin spokesman Dmitry Peskov said China's participation "speaks about the expansion of interaction of the two allies in all the spheres."

Chinese forces have already joined their Russian counterparts in some military exercises.

Chinese warships have drilled with their Russian counterparts in the Pacific Ocean and the Baltic Sea. In summer 2018 Chinese warplanes were in Russia for International Army Games 2018, a multinational event.

August 2018, Chinese forces are taking part in Peace Mission 2018, an exercise organized by the Shanghai Cooperation Organization, a regional bloc led by Russia and China. (It's the first exercise to include all eight SCO members.)

China's Jian-10 fighter jet

But including China in the Vostok exercise hints at a significant geopolitical shift.

"China was seen as the potential threat or target in exercises like Vostok," Alexander Gabuev, an expert on China at the Carnegie Moscow Center, told The New York Times.

"But it is now being invited to join as a friend and even a quasi-ally," Gabuev added. "This is really unprecedented."

The Soviet Union clashed with China along their shared border several times in the 1960s — once in a deadly Chinese raid on a Soviet border outpost that almost kicked off a full-scale war in early 1969.

The Soviet leader Mikhail Gorbachev normalized relations with China in 1989, and some 6 million Russians in Siberia now live alongside roughly 100 million Chinese in northern China, where trade relations have grown.

But eastern Russia's vast expanse and sparse population make it a vulnerable area, and Russians there have expressed frustration with the growing Chinese presence and with concessions to Chinese commercial interests.

Amid heightened tensions with the West, however, Russian President Vladimir Putin has made a concerted effort to build ties with China. Beijing, for its part, has also embraced Russia. Both have done so with an eye on the West.

United States President Donald Trump and Russian President Valdimir Putin.

The two have said they are building a "strategic partnership" and expressed shared opposition to what they describe as a "unipolar" world dominated by the US.

China's defense minister, Gen. Wei Fenghe, went to Moscow early 2018 on his first trip abroad, saying the visit was meant to "let the Americans know about the close ties between the armed forces of China and Russia."

"I am visiting Russia as a new defense minister of China to show the world a high level of development of our bilateral relations and firm determination of our armed forces to strengthen strategic cooperation," Wei said.

That rhetoric and statements about close ties don't mean that Russia has dropped its guard, Gabuev said, noting that Chinese troops at Vostok-18 may be limited to training areas near the countries' shared border with Mongolia, allowing Russian forces deployed elsewhere to carry out exercises designed with China in mind.

The Russian military "is not so naive that it is not preparing a contingency plan," Gabuev told The Times.

This article originally appeared on Business Insider. Follow @BusinessInsider on Twitter.